space-time metric
c squared d tau squared equals the sum from mu and nu equal one to four of g sub mu nu of x d x super mu d x super nu, equals g sub mu nu of x d x super mu d x super nu.
c squared d tau squared equals the sum from mu and nu equal one to four of g sub mu nu of x d x super mu d x super nu, equals g sub mu nu of x d x super mu d x super nu.